Project Summary Astrocytes are the most abundant non-neuronal cell types in the brain. Astrocytes promote neuronal survival, provide neurons with metabolic substrates, help recycle synaptic neurotransmitters, regulate blood flow, buffer extracellular ions, induce or eliminate synapses and regulate synaptic transmission and synaptic plasticity. Most recently, it has been shown that when specific signaling pathways are compromised, astrocytes can become reactive and can start eliminating synapses ultimately; leading to neuronal cell death during both natural aging and age-related neurodegenerative diseases. Our research aims to understand how astroglial activation is initiated in Alzheimer?s Disease (AD). We will focus on this question by studying a gene called granulin (GRN). This gene has been linked to aging in the human cortex and loss of GRN inevitably leads to astroglial activation and a devastating neurodegenerative disease termed Frontal Temporal Lobar Degeneration (FTLD). To begin to tackle this question, we propose investigating the molecular mechanisms of GRN signaling on astroglial activation. The goal of our work is to understand how normal GRN function prevents protein mislocalization in healthy aging human neurons, and how loss of GRN causes these same proteins to mislocalize in neurons: leading to neuronal death in AD and FTLD. Based on recent findings that rodent astrocytes and human astrocytes are genetically and morphologically very different, we will use human cells to study this question and set up a novel 3D human induced pluripotent stem cell (iPSC) organoid system to incorporate human neurons, astrocytes and microglia. We will then analyze the mislocalization of specific protein in GRN positive and GRN knockout organoids. We will also study the signaling differences between GRN positive and GRN knockout astrocytes, and focus on the innate immune complement pathway and how GRN affects this pathway. Collectively, these studies will allow us to identify the factors in astrocytes that promote brain homeostasis, aging and disease.